This invention relates to frequency conversion of laser radiation. In particular, the present invention uses Raman scattering to convert the output of a laser to radiation having a frequency corresponding to a solar Fraunhofer line in the blue region of the electromagnetic spectrum.
There are many applications where it is desirable to communicate through sea water. Because sea water is most transparent in the blue-green region of the electromagnetic spectrum, light falling within this region can be used as a communication medium. Generally, blue-green light is light having a wavelength from 420-540 nm. Light having a wavelength of about 480 nm has optimal transmissibility in sea water. One approach for generating blue-green light is to direct the output of an excimer laser through a Raman cell using atomic vapors such as Pb or Ba. Another technique employs multiple Stokes shifting (i.e. nonresonant vibrational Raman scattering) in molecular gases such as H.sub.2 at several atmospheres of pressure. The main limitation of the former approach is the material problems associated with the high temperature required to maintain the necessary atomic vapor density. The latter approach avoids the disadvantages of the former method, but does not yield a particularly advantageous blue-green wavelength very near the wavelength of maximum optical transmittance in sea water.
A significant problem associated with a blue-green carrier is that the solar background represents a source of noise. In fact, per unit wavelength interval, the solar output is a maximum at approximately 500 nm, which is in the middle of the blue-green portion of the electromagnetic spectrum. Therefore, a need exists for a method of generating a light signal having optimal transmission characteristics in sea-water, while limiting the noise attributable to solar radiation.